In a perfect world, your shoulder joint, elbow, forearm and wrist would fall in line with your right foot (assuming a right handed player). That is what I would call your stroking plane. And it is a worthwhile ideal to work toward.
That said, there are probably more great players who don't do that than do. Recently, someone posted a challenge of the idea of a perfect plane, citing many instances where the upper arm was at significant angle to the "proper" shooting plane. I've been thinking a lot about that post and considering the number of great players--Mosconi, Efren, Bustamante, Alex and Mika, to name a few--who's upper arm tracks inside the plane line. The reason I found that thread so interesting is I discovered I delivered the cue straighter (to my dismay), when I was off plane like the players I mentioned.
In any case, I think the key to a good stroke is posture, not arm position. Good posture will put the arm in a workable position. In this respect, having the rear foot on the shot line, feet angled 45 degrees and shoulders rotated enough to give the arm freedom to swing, are the keys. I believe, if you get yourself into that position, your elbow will find where it needs to be to deliver the cue straight.
Of course, making these changes is hit and miss (literally) if you are doing it alone. Not that you can't, but it's hard to see yourself. Video would help a lot. But, better still, a qualified teacher would probably be a worthwhile investment.